TL;DR:
- UL94 is a flammability test standard, not a material — it classifies how a plastic or insulation material burns, from HB (horizontal) up through V-0, V-1, V-2 to 5VA/5VB.
- HB measures horizontal burn rate; V-0 measures vertical afterflame time. The vertical test is more demanding because the flame rises along the specimen.
- A material that meets V-0 generally satisfies HB, so a drawing that says “HB + V-0” is resolved by the stricter V-0 requirement.
- Transformer insulation varies widely — epoxy glass laminates like FR4 are commonly V-0, while untreated cellulose pressboard and paper are not flame-rated.
- Verify the rating with documents — a UL file number, a test report, or a certificate of conformance — before you accept the shipment.
UL94 flammability ratings classify how a plastic or electrical insulation material burns under a defined test flame, using letters and numbers such as HB, V-0, and 5VA that tell you both the test orientation and the pass criteria. This article explains what the UL94 rating system measures, the exact difference between HB and V-0, how a real project resolved a drawing that asked for both, which ratings transformer insulation materials actually need, and how to confirm a material meets your flammability specification before you order.
What Is a UL94 Flammability Rating?
The Test Method and Rating Hierarchy
UL94, published by UL Standards & Engagement, is the standard test for the flammability of plastic materials and insulation. It divides materials into a hierarchy of ratings. HB is the lowest rating and uses a horizontal specimen; V-0, V-1, and V-2 use a vertical specimen and become progressively more demanding; and 5VA and 5VB are the strictest, adding a plaque test on top of the vertical bar test. Each rating is defined by measurable pass criteria — burn rate, afterflame time, total afterflame time, afterglow, and whether flaming drips ignite a cotton indicator — so a rating is not a vague claim but a set of numbers a laboratory can reproduce. For a buyer, the rating is a shorthand for how the material will behave if it is exposed to a small ignition source in service.
Horizontal Burn vs Vertical Burn
The orientation of the specimen changes what the test measures. In the horizontal burn (HB) test, a bar is held horizontally and lit at the free end; the test records how fast the flame front travels along the bar, with limits on burn rate and self-extinguishing before a marked distance. In the vertical burn test, the bar is held vertically and lit at the bottom, so the flame naturally rises up the specimen and preheats the material above it — a more severe condition. Because heat rises, the vertical test is inherently tougher, which is why a material rated V-0 has demonstrated resistance that a mere HB rating does not require. Understanding this orientation difference is the first step to reading a drawing that mixes the two terms.
HB vs V-0: The Key Difference
What Each Rating Actually Requires
HB (Horizontal Burn) is the baseline. For a material 3.0 mm or thicker, the burn rate must not exceed 40 mm per minute; for thinner material it must not exceed 75 mm per minute, and the flame must self-extinguish before reaching a reference mark. V-0 (Vertical Burn, class 0) is far stricter: each individual afterflame must be 10 seconds or less, the total afterflame across ten flame applications must be 50 seconds or less, afterglow must stop within 30 seconds, and no flaming particle may ignite the cotton indicator below. In other words, HB tolerates a slow horizontal burn, while V-0 demands rapid self-extinguishing in the vertical orientation. The difference is not cosmetic — it is the gap between “burns slowly” and “goes out by itself.”
Why V-0 Implies HB
Because the vertical test is more severe than the horizontal test, a material that passes V-0 will in practice satisfy the HB requirements as well. This is not an automatic equivalence written into the standard, but it is the practical engineering reality: if a material self-extinguishes quickly when held vertically with the flame rising along it, it will not sustain a horizontal burn rate above the HB limits. When a specification lists both “HB” and “V-0,” the correct reading is that the stricter V-0 governs. This is the reasoning SIDA applies when a customer drawing repeats both terms, and it resolves the apparent contradiction without ordering a redundant second test.
A Real Project Case: When Drawings Say “HB + V-0”
The eqmar Clarification Chain
A real transformer insulation project — referred to here as the eqmar project — illustrates the ambiguity. The customer drawing specified both “HB” and “V-0” for an insulation component, listing the two ratings side by side. The clarification followed a short chain: the drawing was flagged, the supplier confirmed the actual grade of the material, and the two ratings were interpreted against the UL94 rules. Rather than treating “HB + V-0” as two separate tests to run, the supplier explained that the material’s V-0 certification already covered the horizontal requirement, because a vertically self-extinguishing material cannot fail the horizontal burn-rate limits.
The Resolution: The Stricter V-0 Governs
The project concluded by resolving to the stricter rating. Because V-0 is more demanding than HB, the material was specified and delivered to V-0, with the horizontal-burn compliance confirmed as a consequence rather than as an additional pass condition. This is the correct and defensible way to handle a dual rating: read the requirement as “V-0 or better,” verify the material against V-0, and state in the response that HB compliance is inherent. For any buyer facing a similar drawing, the lesson is to clarify with the supplier rather than assume two tests are required — which saves lead time and avoids paying for redundant certification.
Which Rating Do Transformer Insulation Materials Need?
Typical Flammability by Material
Transformer insulation spans a wide flammability range, so the right rating depends on the material. Epoxy glass laminates are the most common flame-rated materials: FR4 epoxy glass laminate is typically V-0 because it contains a brominated flame retardant, while G10 (the non-flame-retardant grade) is usually rated HB. Phenolic laminates, such as phenolic cotton cloth laminate, can be supplied in HB or flame-retardant grades depending on the resin. By contrast, untreated cellulose pressboard and insulation paper — transformer pressboard and kraft paper — are not flame-rated materials in the UL94 sense; they are chosen for dielectric performance in oil-filled equipment, where flammability is managed by the insulating oil and enclosure rather than by the solid insulation.
How to Ask the Supplier for Certification
When a customer needs a flammability rating, the request should be explicit: name the rating (for example V-0), the material, and the thickness range, and ask for the supporting document rather than a verbal assurance. A supplier can provide a UL file number, an independent test report, or a certificate of conformance that ties the rating to the exact grade being quoted. SIDA supplies flame-rated epoxy and phenolic laminates with the corresponding documentation, so the rating is traceable to a tested material rather than stated as a marketing claim. Asking for the document up front converts a vague requirement into a checkable commitment.
How to Confirm a Material Meets Your Flammability Spec
Documents, Test Reports, and UL File Verification
Confirmation starts with the paperwork. A UL file number can be looked up in the UL Product iQ database to see the manufacturer, the recognized material, and its assigned rating. A third-party test report states the test method, specimen thickness, and measured pass criteria, so you can see whether the material was tested at a thickness comparable to your part. A certificate of conformance ties a specific production batch to the tested grade. The key check is that the rated material matches the material actually being shipped — a V-0 rating for one grade or thickness does not automatically transfer to a different grade. Before releasing the balance payment, verify the document against the material on the proforma invoice. See what to verify before a bulk insulation order for the full protocol.
Frequently Asked Questions
Is V-0 always better than HB?
For flammability, yes — V-0 is the stricter and safer rating because it demands rapid self-extinguishing in the vertical orientation, while HB only limits the horizontal burn rate. The trade-off is cost and material choice: achieving V-0 usually requires a flame-retardant formulation, which can add cost or slightly affect other properties. If your application only needs a minimal level of flame resistance, HB may be sufficient and more economical, but for electrical insulation near potential ignition sources, V-0 is the common default.
Can a material be both HB and V-0?
Yes, and that combination is normal rather than contradictory. A material that meets V-0 will also satisfy the HB requirements because the vertical test is more severe than the horizontal test. When a drawing lists “HB + V-0,” the correct interpretation is that V-0 governs, and the HB requirement is satisfied as a consequence. This is the point of the eqmar project case above, and it is how suppliers resolve dual ratings without running a redundant horizontal test.
Does transformer pressboard have a UL94 rating?
Not typically. Cellulose transformer pressboard and insulation paper are not flame-rated materials in the UL94 sense; they are cellulose products selected for dielectric strength in oil-immersed equipment, where the insulating oil and tank control fire risk. Flammability ratings such as V-0 apply to solid polymer laminates — epoxy glass, phenolic, and polyester mats — used in dry-type transformers, bushing supports, and insulating structures. If your application needs a flame-rated board, ask for a flame-retardant epoxy or phenolic laminate rather than pressboard.
How do I verify a UL94 rating is genuine?
Look up the UL file number in the UL Product iQ database and confirm the manufacturer, material name, and assigned rating match what you are being quoted. Then check that the test report lists the same material and a thickness comparable to your part, because ratings are thickness-dependent. Finally, make the certificate of conformance a condition of the order, so the rated material is tied to the batch actually shipped. These three steps turn a rating claim into something verifiable before the goods reach your dock.
Summary: Reading Flammability Ratings Correctly
UL94 flammability ratings are a hierarchy, not a single pass/fail flag: HB is the horizontal baseline, V-0 is the demanding vertical self-extinguishing requirement, and 5VA/5VB sit above them. Because the vertical test is more severe, a V-0 material satisfies HB, so a drawing that lists both should be read as “V-0 governs.” Match the rating to the material — epoxy and phenolic laminates are the flame-rated options for transformer insulation, while cellulose pressboard is not — and always verify the rating with a UL file number, a test report, or a certificate of conformance tied to the shipped batch.
For flame-rated epoxy glass and phenolic laminates with traceable documentation, SIDA supplies FR4 epoxy glass laminate and machined components to your flammability specification. Contact SIDA to confirm the rating you need:
- 📞 +86-15958243831
- 📧 jessie.feng@sidanm.com
- 💬 WhatsApp: Chat on WhatsApp
- 🌐 sidanm.com
References
- UL Standards & Engagement. UL 94: Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances. https://www.shopulstandards.com/ProductDetail.aspx?productId=UL94
- UL Solutions. UL Product iQ Database (verify a UL file number). https://iq.ulprospector.com/en/
- International Electrotechnical Commission. IEC 60695-11-10: Fire hazard testing — Test flames — 50 W horizontal and vertical flame test methods. IEC webstore.
- International Electrotechnical Commission. IEC 60695-11-20: Fire hazard testing — Test flames — 500 W flame test methods. IEC webstore.
- ASTM International. ASTM D635: Standard Test Method for Rate of Burning and/or Extent and Time of Burning of Plastics in a Horizontal Position. https://www.astm.org/d0635-22.html
- UL Solutions. UL 94 flammability ratings explained. https://www.ulprospector.com/knowledge/6683/pe-ul94-flammability-ratings-explained/
